1. Field of the Invention
The present invention relates to combustion phase control of an internal combustion engine. In particular, the present invention is a method for detecting abnormal pre-ignition combustion at low rpm and at high load in a combustion chamber of an internal combustion engine. More specifically, but not exclusively, the invention is a method applied to a “downsized” spark-ignition engine functioning under very high loads.
2. Description of the Prior Art
This type of engine has at least one cylinder comprising a combustion chamber defined by the internal lateral wall of the cylinder, by the top of the piston that slides in this cylinder and by the cylinder head. Generally, a carbureted mixture is enclosed in this combustion chamber and is subjected to a compression step and then a combustion step under the effect of controlled ignition by a spark plug. These steps are collectively described by the term “combustion phase” in the following description.
It has been possible to confirm that this carbureted mixture can be subjected to different types of combustion and that these types of combustion are the source of different pressure levels, as well as mechanical and/or thermal stresses, of which some can seriously damage the engine.
The first combustion, called conventional combustion or normal combustion, is the result of the propagation of the combination of a carbureted mixture compressed by a prior engine compression step. This combustion propagates normally along a flame front starting from the spark generated by the spark plug and involves no risk of harming the engine.
Another type of combustion is combustion with “knocking”, which results from undesirable self-ignition in the combustion chamber. Thus after the compression step of the carbureted mixture, the spark plug is actuated to allow ignition of the carbureted mixture. Due to the effect of the pressure generated by the piston and the heat released by the start of combustion of the carbureted mixture, forceful and localized auto-ignition of part of the compressed carbureted mixture occurs before the arrival of the flame front issued from the ignition of the carbureted mixture by the spark plug. Knocking leads to a local increase of the pressure and the temperature and can cause destructive effects on the engine and mainly at the piston, if repeated.
Finally, another type of combustion is abnormal combustion due to pre-ignition of the carbureted mixture before the spark plug initiates ignition of the carbureted mixture present in the combustion chamber.
This abnormal combustion affects engines resulting from an operation of “miniaturization,” better known by the English term “downsizing.” This operation tends to decrease the size and/or the cylinder of the engine while maintaining the same power and/or the same torque as conventional engines. Generally, this type of engines is mainly of the gasoline type and is highly supercharged.
It has been possible to confirm that this type of abnormal combustion occurs with high loads and generally during low rpm of the engine when the heat of combustion of the carbureted mixture is not optimum due to knocking. Taking into account the strong pressures and the elevated temperature reached in the combustion chamber by supercharging, starting of abnormal combustion can occur, sporadically or in a continuous manner, before the moment where the ignition of the carbureted mixture by the spark plug takes place. This combustion is characterized by a first phase of flame propagation that is timed too late in comparison to that of conventional combustion. This propagation phase can be interrupted by self-ignition which will involve a large majority of the carbureted mixture present in the combustion chamber which is much greater than that in the case of knocking.
In the case where this abnormal combustion is produced repetitively, from engine cycle to engine cycle, and occurs starting from a hot point of the cylinder, for example, it is called “pre-ignition.” If this combustion occurs in a violent manner, randomly and sporadically, it is called “rumble” (“pre-ignition”).
This latter abnormal combustion involves very elevated pressure levels (120 to 250 bars), as well as an increase of the thermal transfers that can involve partial or total destruction of the moving engine parts, like the pistons or the rods.
The general methodology for processing this type of abnormal combustion is diagrammed in FIG. 1. The first step is a prevention phase (PP) for limiting to a maximum the chances of appearance of the phenomenon. Then when prevention is not adequate to prevent the phenomenon, a detection phase (PD) is performed for determining whether or not there is a need to intervene in the cycle with a corrective phase (PC) when pre-ignition has been detected.
The detection phase (PD) comprises a signal acquisition phase followed by a signal processing phase making it possible to detect the appearance of pre-ignition at high load to describe and to quantify the pre-ignition at high load.
Patent Application EP 1,828,737 discloses a method for detecting the appearance of pre-ignition at high load of the “rumble” type. This method is based on the measurement of a signal relative to the progress of the combustion and a comparison to a signal threshold. The presence of an abnormal combustion of the “rumble” type in the combustion chamber is detected when the signal amplitude exceeds that of the signal threshold in a significant manner. According to this method, the signal threshold corresponds to the amplitude of the signal produced at the time of combustion with knocking or at the time of normal combustion.
However, according to this method, the detection thus implemented does not make it possible to act during the same cycle as the detection. The corrective actions of this type of pre-ignition are only implemented for the phenomenon that can seriously damage the integrity of the engine.
The method described in French Patent 2,897,900 makes possible more rapid action after detection of the pre-ignition. It is possible to act during the course of the same cycle as the detection cycle of the phenomenon. In order to do this, the signal threshold is calculated in advance, before the operation of the engine, and then is stored in calculator data tables known as maps.
However, the use of maps does not make it possible to detect, at any time, that is in real time, the start of such a phenomenon. Because of this fact, it is always possible that the detection may occur too late. In addition, no quantification of the trend of the pre-ignition can be implemented. Thus, the necessity to not to apply a correction action rests solely on the comparison of the two amplitudes at a given moment in time. Still, such a phenomenon can actually start, then stop, without involving damage to the engine and thus not require a corrective phase.